page_title: Solar irradiance
section_title: Projection effect
hierarchical_section_title: Solar irradiance / Irradiance on Earth's surface / Projection effect
caption_attribution_description: English: How directness of sunlight causes warmer weather and how it effects climates.
context_page_description: Solar irradiance is the power per unit area, received from the Sun in the form of electromagnetic radiation as reported in the wavelength range of the measuring instrument.
  Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment, during that time period. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation.
  Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering.
  Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes.
  Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the sun above the horizon, and atmospheric conditions.
  Solar irradiance affects plant metabolism and animal behavior.
  The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and in climate modeling and weather forecasting.
context_section_description: Insolation onto a surface is largest when the surface directly faces (is normal to) the sun. As the angle between the surface and the Sun moves from normal, the insolation is reduced in proportion to the angle's cosine; see effect of sun angle on climate.
  In the figure, the angle shown is between the ground and the sunbeam rather than between the vertical direction and the sunbeam; hence the sine rather than the cosine is appropriate.  A sunbeam one mile (1.6 km) wide arrives from directly overhead, and another at a 30° angle to the horizontal. The sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the angled sunbeam spreads the light over twice the area.  Consequently, half as much light falls on each square mile.
  This 'projection effect' is the main reason why Earth's polar regions are much colder than equatorial regions. On an annual average the poles receive less insolation than does the equator, because the poles are always angled more away from the sun than the tropics, and moreover receive no insolation at all for the six months of their respective winters.
caption: Projection effect: One sunbeam one mile wide shines on the ground at a 90° angle, and another at a 30° angle. The oblique sunbeam distributes its light energy over twice as much area.
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